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Aggregating Loopback addresses


All documents/book always reference that LSP will be broken if you summarize PE Loopback addresses used as update-source in MP-BGP sessions. I would like to run by this forum one scenario and see what you guys have to say.

Let's say you're running OSPF as your backbone IGP, which is not far from reality :-)

Let's say you have egress PE router with loopback IP address

At some point in your backbone you aggregate this lookback IP to Let's see what happens now...Aggregating router will create a route pointing to Null interface, this route will be considered connected and thus advtertised with implicit-null label value. This will cause PHP bevahior and a premature exposure of VPN label, which will of course break the LSP.

Now what if you configure "no discard-route internal" under the OSPF configuration on the aggregating router?

In this case Null pointing route will not be created, so there will be no implicit-null value label advertised for Will LSP be broken in this case?

This scenario is not applicable for ISIS which to my knowledge does not have ability to suppress the installation of Null route at aggregation point.




Re: Aggregating Loopback addresses


sorry to inform you that the LSP will still be broken. LDP will advertise imp-null. The LIB entry on the ABR with the "area range" will look like this, if is the summary:

tib entry:, rev 58(no route)

local binding: tag: imp-null

remote binding: tsr:, tag: 17

The underlying reason: the ABR needs to perform a lookup, because forwarding to different subnets contributing to the summary will not be possible otherwise.

Hope this helps! Please rate all posts.

Regards, Martin

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